Switching power amplifiers often are afflicted with non-linearities due to the switching behavior of the output stage. The exact nature of these non-linearities not only depends on the type of power semiconductor(s) used, but also on the connected circuit(s) at load and driving side. In precision power applications, such as for example motion control and gradient amplifiers for magnetic resonance imaging (MRI), compensation of these non-linearities is mandatory because a feed-forward path is in most cases used in the control system.
US 2006/208798 A1 discloses a method of operating a class D amplifier output stage that compensates for nonlinearity introduced by a residual load current during the dead time in the switching of the output stage. The amplifier output stage includes an input, a gate driver circuit, two output transistors, an output, and a current sensing circuit. The transistors are serially connected between the terminals of a power supply. A residual load current flows through the transistors when they are switched off. The gate driver circuit increases or decreases the duty cycles of signals driving the transistors based on the direction of the residual load current flowing through the transistors, thereby causing the duty cycle of the amplifier output to remain substantially constant and equal to the duty cycle of the amplifier input.
As shown in US 2006/208798, the accuracy that can be obtained by using (only) the direction of the load current for compensation is limited. To obtain better results, a model-based approach can be used, for example using look-up tables to predict the system behavior over the next time cycle of interest.
A possible method to fill such a look-up table is to repeatedly simulate the circuit off-line for all combinations of the parameters mentioned above and store the resulting average voltage in a look-up table. An appropriate compensation of the switch timings can be found by using look-up table entries and, if needed, use interpolation to find an approximation of the circuit behavior for exactly the combination of circuit states which has been measured by the controller at the beginning of the control time interval. Such a solution is possible in principle, but will, if implemented in full, need considerable memory resources. Even with the abundance of cheap memory devices available today, such an approach is limited in accuracy and the number of state variables which can be used due to the exponential growth laws inherent in such an approach.